Except as required by law or subject to contractual obligations (e.g. Udacity Enterprise offerings), you have a 7-day window from the day you receive access to the program (the first day your classroom opens) to unenroll and request a refund.
If your class term has already started and you are within the 7-day refund period, you can cancel your Nanodegree and get a refund directly from your classroom.
If you want to cancel and unenroll from your Nanodegree program before the term starts, you can request a refund via this form.
"...you hereby acknowledge and agree that Udacity will not offer refunds on any fees and charges related to your purchase of any products or Services. This includes any partially used or unused periods for which you have already paid. We do not guarantee refunds for lack of usage or dissatisfaction."